Added enemy avatar.
[hitbox] hitbox:register() with no paramaters uses self.class
Minor spacing issues.
[avatar] Hitbox class may change perr avatar.
Print message when avatar are considered hit by atk.
Added message module.
[message] Useful and cool-looking API.
[main] Added message receiver that kills avatars.
[avatar] Atk hit kills enemies... but hitbox remains...
Enemy killing kills hitboxes too.
Merge branch 'enemy'